This invention relates to methods of depositing by pulsed DC reactive sputtering an additive containing aluminium nitride film. The invention relates also to the films themselves and to piezoelectric devices comprising these films.
There is interest in the production of AlN thin films, not least because of their piezoelectric properties. A potentially important application is in Bulk Acoustic Wave (BAW) frequency resonating devices. BAW devices consist of a resonating piezoelectric layer (usually aluminium nitride) sandwiched between two electrodes. This is a technology enabler for the mobile communications industry, as they can be used to manufacture small, cheap precision filters with high rejection, low loss and very low temperature drift. Sputtered aluminium nitride is widely used in the manufacture of BAW filters, mainly because of its relatively high piezoelectric constant, which is the highest amongst common tetrahedral bonded binary semiconductors. However, a drawback is that aluminium nitride films inherently have a rather low electromechanical coupling coefficient which sets a limit of the achievable band width of aluminium nitride containing filters.
In order to improve the piezoelectric properties of aluminium nitride thin films, it has been suggested to incorporate metal additives such as Sc, Y, Ti, Cr, Mg and Hf. For example, scandium may be incorporated into the alloy at the expense of aluminium. Because the Sc—N bond is 0.35 A longer than the Al—N bond length of 1.9 A, a stress is created in the film owing to this difference in bond length. As a consequence of the variable bond length, the alloy material becomes softer. However, with the larger unit cell there is a significant improvement in the electromechanical coupling coefficient. This can be seen in FIG. 1, which shows electromechanical coupling coefficient as a percentage of stress for Al93.9Sc6.1N in comparison to pure aluminium nitride films. At zero film stress, the Al93.9Sc6.1N film exhibits a coupling coefficient of about 8% in comparison to a coupling coefficient of 6.2% for the pure aluminium nitride film. This represents a relative improvement in coupling coefficient of about 30%. It is understood that when the composition is expressed in the form Al100-xScxN, the values 100-x and x are expressed as percentages, and x as a percentage can be equated to 0.0x in stoichiometric chemical terms.
It can also be seen in FIG. 1 that a higher coupling coefficient is achieved with a more tensile film. However, highly stressed films are not suitable for large scale BAW manufacture, because they are liable to crack and peel. This can result in problems with regard to reliability later on in the manufacturing process. Another problem that has been observed with AlScN films thicker than around 300 nm, is that they tend to suffer from degradation in build quality with increasing scandium content. This manifests as surface roughness due to the formation of disoriented grains protruding from the surface of the film. In the ternary nitride Al100-xScxN, there are several competing stable phases. However, the wurtzite Al100-xScxN form is in a non-equilibrium state. Hence, small variations in stress or scandium concentration, for example at a grain boundary, may nucleate an alternative crystal orientation with relative ease. For example, in FIG. 2 we present SEM images of a typical (a) 1.5 micron aluminium nitride film and (b) 1.5 micron Al94Sc6N film deposited using the same PVD deposition parameters. Whilst the aluminium nitride film shown in FIG. 2(a) is smooth, defect free and relatively featureless, the film formed from the ternary scandium alloy shown in FIG. 2(b) has a high density of pyramidal crystallites embedded in the film. These defects serve to reduce the coupling coefficient and quality factor of the films. Additionally, these defects cause problems for further downstream processing such as film lithography/etching and the deposition of subsequent layers on top of the film. Despite the significant defect levels, the AlScN film exhibits good c-axis orientation and the measured XRD (0002) FWHM (full width half maximum) of less than 1.50 are comparable with the results for pure aluminium nitride films. This confirms that a sputtered AlScN film could be an excellent candidate for the manufacture of high performance BAW filters, provided a method could be found to reduce the defect levels. A defect level of less than 20 per 100 square microns is required in order to contemplate high volume, commercial BAW production. A further proviso for eventual commercialisation would be that the method could be carried out in an economically viable fashion.